The present invention relates to a motor control method and apparatus, a time recorder having the motor control apparatus and an impact-type printing apparatus.
Conventional card printing devices, such as many time recorders or xe2x80x9cpunch clocksxe2x80x9d, employ an inexpensive direct current motor as a means for drawing in a time card and pulling it to the correct position for printing in a printing column, for example.
In the case of a time recorder, when a time card is inserted into a card insertion inlet, the card is detected by a sensor, triggering the motor that draws the card in, relative to a printing means, to a position that corresponds with the current date. The card is then stopped and printed by a printing head.
Precise control of the card stopping position is critical to ensure that the card is properly printed. In the prior art this has been accomplished by braking by applying voltage in the reverse direction to the motor when the card is a predetermined distance away from the target position thereby rapidly decelerating the forward movement of the card, bringing it, theoretically, to a stop at the target position.
However, the control method employed by the prior art has several shortcomings. When the electrical current applied to the card drive motor is abruptly reversed, enormous stresses exerted on the motor deteriorate its durability. Furthermore, calculation of a precise stopping distance of the card is inherently inexact, particularly at high card speeds because it is difficult to predict the stopping position of the direct current motor used to move the card. This is a problem shared in common by any system employing a direct current motor requiring an accurate stop position from a high operating speed.
Therefore, in order to resolve some of these problems, applicant has proposed in Japanese Patent Application No. 9-316824 stop control which is accomplished through an intermediate reduction in the speed of the card drive motor. The card drive motor is initially operated at a high speed until the card is relatively close to the target position, and then switched to a lower speed allowing the card to decelerate to a lower speed, from which it is easier to stop accurately. The rotational speed of a DC motor used as a card drive motor is detected while drawing in a card at a high speed. The time required to stop the card is calculated from that speed, and the timing of the switch from high to low speed drive is adjusted accordingly.
In practice however, this approach is imperfect. In order to reduce the time it takes to feed a card, the card drive motor must operate at a high speed for as long as possible during the feeding of the card. As shown in FIG. 5, extending high speed drive until just prior to the point at which the card must be stopped does not leave sufficient time for the card to decelerate on its own to a stabilized velocity equal to that of the low speed drive. This means that the card must be abruptly stopped from some speed higher than that defined by the low speed drive, causing enormous stresses to the motor during braking causing deterioration of the motor and reducing the accuracy of the calculated stop position.
Furthermore, as shown in FIG. 6, switching to low speed drive sufficiently early in the feeding of the card to allow it to be stabilized at equilibrium with the low speed drive increases card feed time, reducing performance. These problems arise not only for motors for time recorders but for any direct current motor requiring an accurate stop position in a short period of time.
Motor control presents a similar problem when applied to a printing apparatus such as that found in a time recorder. Prior art impact printing systems have used stepping motors, making print head feed control easy to carry out, but relatively expensive. Direct current motors, have also been used, subject to the limitation that the scanning speed of the print head is likely to vary, affecting print quality. For example, if the drive is running on a partially discharged battery, scanning speed may fall. Attempts to compensate for this by speeding up the direct current motor are likely to result in transient overspeed, also degrading print quality as described above. Employing a variable voltage driver circuit to stabilize scanning is another expensive solution. An impact print head found in existing time recorders or xe2x80x9cdot-matrixxe2x80x9d printers requires a relatively constant scanning speed to ensure proper timing in the actuation of the impact pins. This is exacerbated by expected variations in machining accuracy of existing head scanning mechanisms and the operational environments in which they are used.
When the moving speed of the printing head is accelerated the striking duration of the printing pin is restricted and print darkness is deteriorated. Therefore, in order to prevent the printing darkness from being deteriorated, it is necessary to set a sufficient striking duration by retarding the speed of the printing head.
As shown in FIG. 18, the speed of a moving impact print head, as measured by the output signal of a sensor for detecting rotation of the driving motor, is fixed so that there is sufficient time for the impact pins to be turned xe2x80x9conxe2x80x9d during which each impact pin extends from its rest position to make a printing impact, and also for the impact pins to be turned xe2x80x9coffxe2x80x9d during which each pin is retracted and returned to its rest position. However as shown in FIG. 19, if the period of the output signal of the sensor for detecting rotation of the driving motor is short (indicating that the print head is fast) there is not sufficient time for the impact pins to be retracted completely, reducing the quality of the subsequent printing impact.
Furthermore, use of existing impact print heads designed for printing on a cylindrical platen presents special problems when used to print on a platen with a different shape, such as a flat plane shaped platen found in some time recorders. FIGS. 12(a) and 12(b) show a print head having a plurality of pins xe2x80x9caxe2x80x9d through xe2x80x9cgxe2x80x9d of a printing head B arranged in a line perpendicular to the length of the cylindrical platen A. The head shown has, by varying the angles of the printing pins, been designed to print on the curved surface of the cylindrical platen. Simultaneous operation of the pins, therefore, will produce impacts on the printed object P such as paper along the center line as shown in FIG. 12(c). The center line is referred to as the column direction and the head B scans in the row direction along the axis of the cylindrical platen A.
However, when the curved surface is replaced with one of a different shape such as the plane shaped platen D shown in FIGS. 13(a) and 13(b), simultaneous operation of the pins will instead produce the uneven xe2x80x9czig-zagxe2x80x9d appearance shown in FIG. 13(c). Printing accuracy is deteriorated. Though a process of trial-and-error wherein the distance between the pins and time card C on the platen D are adjusted may improve the alignment of the impact positions somewhat, the result is generally unsatisfactory and the print quality is reduced. Redesigning the printing head to accommodate flat surfaces is undesirable because it is expensive, and the finished product will have the same disadvantage of being usable only for one type of platen.
Accordingly, there is still a need for a motor control method and an apparatus that overcomes the limitations of direct current motors in devices that feed and print cards to provide accurate speed and stop control without causing premature motor failure or transient speed variations.
There is also a need for a time recorder having a card feeder and an impact printing system incorporating the motor control apparatus that minimizes card feed time and allows printing on multiple platen shapes.
According to an embodiment of the invention, there is provided a motor control method for accurately stopping an object moved by a motor at a predetermined target position. The motor control method comprises the steps of driving the motor at a first predetermined speed until the object is a first predetermined distance from the predetermined target position and then carrying out primary braking to decelerate the motor to a second predetermined speed lower than the first desired speed. The motor is then driven at the lower speed until the object reaches a second predetermined distance from the target position. Secondary braking is then carried out to stop the object precisely at the target position. Thereby, the time period required for accurately stopping the object on target is minimized and the motor is accurately stopped.
In another embodiment the motor is temporarily stopped at midway in order to calculate the required first and second predetermined distances from the target position required to accurately stop an object at the target position.
In a further embodiment the driving speed of the motor is detected while primary braking is carried out. Thereby, the motor is accurately braked to the second predetermined speed.
In another embodiment, a motor control apparatus employing the motor control method of the invention is provided capable of stopping an object swiftly and accurately at a predetermined target position.
In a further embodiment, such a motor control apparatus is applied to a time recorder according to the invention in which a time card can be stopped accurately at a predetermined target position.
In another embodiment, shift amounts of impact positions of the respective printing pins in the row (horizontal) direction relative to a specific platen which is actually used when the plurality of printing pins are driven at the same timing, are previously stored to storing means, striking timings of the respective printing pins are controlled by controlling means based on a moving speed of the printing head in the row direction and the shift amounts of the impact positions of the respective printing pins in the row direction and accordingly, the impact positions of the printing pins can be aligned in a vertical arrangement. Therefore, deterioration in printing accuracy can be prevented, the deterioration being of the type caused in the case in which the printing operation is carried out by using a dot impact type printing head developed for a design platen with a specific platen having a shape different from the shape of the platen used. Further, general use performance of the dot impact type printing head developed for the design platen is enhanced. That is, the problem in which the dot impact type printing head developed for the design platen can be used only for the design platen is resolved.
In a further embodiment according to the invention, the motor is controlled such that the moving speed of the printing head in the row direction becomes a previously set predicted moving speed and the striking timings of the respective printing pins are controlled by the controlling means based on the predicted moving speed and the shift amounts of the impact positions of the respective printing pins in the row direction. By such a constitution, control of arranging the impact positions of the printing pins in the vertical arrangement can be facilitated. Therefore, there can be prevented the deterioration in the printing accuracy caused in the case in which the printing operation is carried out by using the dot impact type printing head developed for a design platen with the specific platen having a shape different from the shape of the platen used on the rear side of the printed object. Further, the general use performance of the dot impact type printing head developed for the design platen is enhanced. That is, the problem in which the dot impact type printing head developed for the design platen can be used only for the design platen is resolved.
In another embodiment, according to the invention, the striking timings of the respective printing pins are controlled by the controlling means based on a moving distance of the printing head in the row direction and the shift amounts of the impact positions of the respective printing pins in the row direction. By such a constitution, the problem similar to the above-described can be resolved and control of aligning the impact positions of the printing pins in the vertical arrangement can be facilitated with no necessity of calculating the moving speed of the printing head.
In a further embodiment, the striking duration time periods of the printing pins are controlled in accordance with the moving speed of the printing head in the row direction. In this way, the printing pins are driven for optimum striking duration time periods and the desired printing operation can be carried out even when the moving speed of the printing head in the row direction is changed by various factors of the motor. Therefore, there can be resolved the problem in which the operation successively proceeds to a next striking duration time period in a state in which the printing pin has not been pulled back to a set position, the printing pins cannot carry out correct ON and OFF operation and desired printing operation cannot be carried out.
Further, the striking duration time periods of the printing pins are stored in correspondence with the moving speed of the printing head in the row direction. In this way, the problem similar to the above-described problem can be resolved. The printing pins can be driven by reading the striking duration time periods immediately in accordance with the moving speed of the printing head in the row direction and accordingly, swift control is realized. That is, there can be resolved the problem of increasing a control time period caused in the case of calculating the striking duration time periods of the printing pins by, for example, a calculating operation.
Further, there is provided detecting means of the moving speed of the printing head in the row direction. In this way, a further swift control is realized.
Further, there is calculated the average value of the newest plurality of moving speeds detected by the speed detecting means and the striking duration time periods of the printing pins are controlled in accordance with the average value of the moving speeds. In this way, the printing pins can be controlled by further accurate striking duration time periods.